The present invention relates to a probe head for a coordinate-measuring instrument wherein deflectable probe-mounting structure defines a three-dimensional coordinate system so that a work-contacting probe pin can be deflected in all of three coordinate directions in space.
The coordinate-measuring instrument serves to move the probe head in a three-dimensionally controlled manner until the pin of the probe contacts the object to be measured at a predetermined place, the space coordinates being fixedly established with high precision at the instant of contact with the workpiece (object). An associated computer determines desired measurements of the workpiece, from the space coordinates of different contact locations on the workpiece.
A large number of probe heads for coordinate-measuring instruments are known, but all of these are subject to the limitation that they can be used only in a given orientation in space.
Thus, West German Pat. No. 2,242,355 and its corresponding U.S. Pat. No. 3,869,799 disclose a probe head having a probe-mounting system which consists of a torsionally rigid sequence of play-free and friction-free linear suspensions, wherein each suspension accommodates deflection in a different one of the coordinate directions of a three-dimensional coordinate system. This probe head is used in a vertical orientation in space which corresponds to the z-axis. Provision is made for weight equalization in the z-direction, in order to relieve the suspended probe from gravitionally loaded contact with the workpiece. This equalization is effected via a spring which is arranged between that part of the probe head which is secured to the machine and the part of the suspension system which is adapted to receive the probe pin. The force of this spring is adjustable via a motor which effects z-axis displacement of the point of attachment of the spring to the machine until a zero indicator (within the z-axis suspension) becomes operative to stop the spring-displacement motor, i.e., when the zero position is reached.
This known probe head can be used only in a single orientation in space since it is only in the z-direction that the spring can provide the weightcounterbalancing adjustment which is necessary when making a change from one to another probe pin. As discussed in West German Pat. No. 2,242,355, it is desirable that numerous different probe pins be available for selective connection to the probe head; for a given mounted position of the workpiece, an adequate inventory of different replaceable probe pins and probe-pin configurations makes it possible to make work-contact measurements at workpiece locations which would otherwise be inaccessible for a single probe pin or probe-pin configuration.
Nevertheless, many measurement tasks can be handled only with great difficulty, if at all, with a probe head which can be inserted only in one orientation in space. These include, for example, the task of measuring in the inside of a workpiece when the surface to be contacted lies so far within the workpiece that it can be reached, if at all, only with a very long probe pin.
U.S. Pat. No. 4,364,179 describes a probe head which accepts probe-pin insertion in a horizontal orientation in space. In this case, the probe pin is secured to a cylindrical body which is mounted for horizontal displacement within the probe-head housing. This cylindrical body is suspended by a Hooke's joint via a bushing in an outer part of the housing in such manner that it can be articulated with limited universal action about two axes which are mutually perpendicular.
In this horizontally oriented case, taring of the probe head is effected manually by shifting weights on the cylindrical body so that rotational moments are equal on both sides of the horizontal rotary axis defined by the Hooke's joint. Such taring is effective only for a single axis, so that the probe pin can be inserted only in its horizontal orientation in space. Furthermore, such taring is cumbersome and cannot be automated, and it suffers from the disadvantage that the mass to be moved at workpiece contact is increased by the taring weights, thus detrimentally affecting the obtainable precision.